# Full text of "Text Book Of Mechanical Engineering"

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```Head, Pressure, and Velocity Energy.

711

To connect head and velocity:  a water particle of weight zev
while at A, Fig. 694, has a potential energy #/H, and when fallen

to B a kinetic energy of - .   Neglecting friction and other losses,.

and 7'

nearly.

When water flows steadily between reservoirs kept at constant
level, any portion of water will, neglecting friction and viscosity,
be in possession of an unvarying amount of energy, which may
be due to head, pressure, velocity, or all three. In Fig. 695, a
pressure column A falls short of level c, a portion of the head
energy having become kinetic; and the total head J^ consists

P                                   v*1

of H due to unexpended fall, -= due to pressure, and — due to

G             r                    2g

velocity.    Multiplying each by w gives the respective energy, and
the energy in one Ib. of water

P         7/2

5 + T

H

An interesting experiment, due to Froude, is given in Fig. 696.
Two tanks, A and B, have discharge pipes c and D, the former
throttled at E, and the latter expanded at F, causing the velocity
energy to become respectively greater or less than at the tank
mouth, as shewn by pressure columns. Further, the horizontal
pressures at E and at F exactly balance, and there is no tendency
to move the pipe.

The Jet Pump. — With sufficient throttling, the pressure
may be reduced below that of the atmosphere, the principle
employed in Prof. Jas. Thomson's jet pump, Fig. 697. Water,.
under a good head, enters pipe D, and passing through the nozzle
at a high velocity, produces a partial vacuum around it. More
water entering at A to fill the gap, the combined streams dis-
charge at B, and thus a field may be drained or other work
performed.

Discharge of Water from Orifices. — A tank being
emptied through an orifice near its bottom, the volume of water```